Defining Constraints and Indexes — SQLAlchemy 0.8 Documentation

# SQLAlchemy 0.8 Documentation

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Release: 0.8.4 | Release Date: December 8, 2013

# Defining Constraints and Indexes

This section will discuss SQL constraints and indexes. In SQLAlchemy the key classes include `ForeignKeyConstraint` and `Index`.

## Defining Foreign Keys

A foreign key in SQL is a table-level construct that constrains one or more columns in that table to only allow values that are present in a different set of columns, typically but not always located on a different table. We call the columns which are constrained the foreign key columns and the columns which they are constrained towards the referenced columns. The referenced columns almost always define the primary key for their owning table, though there are exceptions to this. The foreign key is the “joint” that connects together pairs of rows which have a relationship with each other, and SQLAlchemy assigns very deep importance to this concept in virtually every area of its operation.

In SQLAlchemy as well as in DDL, foreign key constraints can be defined as additional attributes within the table clause, or for single-column foreign keys they may optionally be specified within the definition of a single column. The single column foreign key is more common, and at the column level is specified by constructing a `ForeignKey` object as an argument to a `Column` object:

    user_preference = Table('user_preference', metadata,
        Column('pref_id', Integer, primary_key=True),
        Column('user_id', Integer, ForeignKey("user.user_id"), nullable=False),
        Column('pref_name', String(40), nullable=False),
        Column('pref_value', String(100))
    )

Above, we define a new table `user_preference` for which each row must contain a value in the `user_id` column that also exists in the `user` table’s `user_id` column.

The argument to `ForeignKey` is most commonly a string of the form <tablename>.<columnname>, or for a table in a remote schema or “owner” of the form <schemaname>.<tablename>.<columnname>. It may also be an actual `Column` object, which as we’ll see later is accessed from an existing `Table` object via its `c` collection:

    ForeignKey(user.c.user_id)

The advantage to using a string is that the in-python linkage between `user` and `user_preference` is resolved only when first needed, so that table objects can be easily spread across multiple modules and defined in any order.

Foreign keys may also be defined at the table level, using the `ForeignKeyConstraint` object. This object can describe a single- or multi-column foreign key. A multi-column foreign key is known as a composite foreign key, and almost always references a table that has a composite primary key. Below we define a table `invoice` which has a composite primary key:

    invoice = Table('invoice', metadata,
        Column('invoice_id', Integer, primary_key=True),
        Column('ref_num', Integer, primary_key=True),
        Column('description', String(60), nullable=False)
    )

And then a table `invoice_item` with a composite foreign key referencing `invoice`:

    invoice_item = Table('invoice_item', metadata,
        Column('item_id', Integer, primary_key=True),
        Column('item_name', String(60), nullable=False),
        Column('invoice_id', Integer, nullable=False),
        Column('ref_num', Integer, nullable=False),
        ForeignKeyConstraint(['invoice_id', 'ref_num'], ['invoice.invoice_id', 'invoice.ref_num'])
    )

It’s important to note that the `ForeignKeyConstraint` is the only way to define a composite foreign key. While we could also have placed individual `ForeignKey` objects on both the `invoice_item.invoice_id` and `invoice_item.ref_num` columns, SQLAlchemy would not be aware that these two values should be paired together - it would be two individual foreign key constraints instead of a single composite foreign key referencing two columns.

### Creating/Dropping Foreign Key Constraints via ALTER

In all the above examples, the `ForeignKey` object causes the “REFERENCES” keyword to be added inline to a column definition within a “CREATE TABLE” statement when `create_all()` is issued, and `ForeignKeyConstraint` invokes the “CONSTRAINT” keyword inline with “CREATE TABLE”. There are some cases where this is undesireable, particularly when two tables reference each other mutually, each with a foreign key referencing the other. In such a situation at least one of the foreign key constraints must be generated after both tables have been built. To support such a scheme, `ForeignKey` and `ForeignKeyConstraint` offer the flag `use_alter=True`. When using this flag, the constraint will be generated using a definition similar to “ALTER TABLE <tablename> ADD CONSTRAINT <name> ...”. Since a name is required, the `name` attribute must also be specified. For example:

    node = Table('node', meta,
        Column('node_id', Integer, primary_key=True),
        Column('primary_element', Integer,
            ForeignKey('element.element_id', use_alter=True, name='fk_node_element_id')
        )
    )

    element = Table('element', meta,
        Column('element_id', Integer, primary_key=True),
        Column('parent_node_id', Integer),
        ForeignKeyConstraint(
            ['parent_node_id'],
            ['node.node_id'],
            use_alter=True,
            name='fk_element_parent_node_id'
        )
    )

### ON UPDATE and ON DELETE

Most databases support cascading of foreign key values, that is the when a parent row is updated the new value is placed in child rows, or when the parent row is deleted all corresponding child rows are set to null or deleted. In data definition language these are specified using phrases like “ON UPDATE CASCADE”, “ON DELETE CASCADE”, and “ON DELETE SET NULL”, corresponding to foreign key constraints. The phrase after “ON UPDATE” or “ON DELETE” may also other allow other phrases that are specific to the database in use. The `ForeignKey` and `ForeignKeyConstraint` objects support the generation of this clause via the `onupdate` and `ondelete` keyword arguments. The value is any string which will be output after the appropriate “ON UPDATE” or “ON DELETE” phrase:

    child = Table('child', meta,
        Column('id', Integer,
                ForeignKey('parent.id', onupdate="CASCADE", ondelete="CASCADE"),
                primary_key=True
        )
    )

    composite = Table('composite', meta,
        Column('id', Integer, primary_key=True),
        Column('rev_id', Integer),
        Column('note_id', Integer),
        ForeignKeyConstraint(
                    ['rev_id', 'note_id'],
                    ['revisions.id', 'revisions.note_id'],
                    onupdate="CASCADE", ondelete="SET NULL"
        )
    )

Note that these clauses are not supported on SQLite, and require `InnoDB` tables when used with MySQL. They may also not be supported on other databases.

## UNIQUE Constraint

Unique constraints can be created anonymously on a single column using the `unique` keyword on `Column`. Explicitly named unique constraints and/or those with multiple columns are created via the `UniqueConstraint` table-level construct.

    meta = MetaData()
    mytable = Table('mytable', meta,

        # per-column anonymous unique constraint
        Column('col1', Integer, unique=True),

        Column('col2', Integer),
        Column('col3', Integer),

        # explicit/composite unique constraint.  'name' is optional.
        UniqueConstraint('col2', 'col3', name='uix_1')
        )

## CHECK Constraint

Check constraints can be named or unnamed and can be created at the Column or Table level, using the `CheckConstraint` construct. The text of the check constraint is passed directly through to the database, so there is limited “database independent” behavior. Column level check constraints generally should only refer to the column to which they are placed, while table level constraints can refer to any columns in the table.

Note that some databases do not actively support check constraints such as MySQL.

    meta = MetaData()
    mytable = Table('mytable', meta,

        # per-column CHECK constraint
        Column('col1', Integer, CheckConstraint('col1>5')),

        Column('col2', Integer),
        Column('col3', Integer),

        # table level CHECK constraint.  'name' is optional.
        CheckConstraint('col2 > col3 + 5', name='check1')
        )

    sqlmytable.create(engine)
    CREATE TABLE mytable (
        col1 INTEGER  CHECK (col1>5),
        col2 INTEGER,
        col3 INTEGER,
        CONSTRAINT check1  CHECK (col2 > col3 + 5)
    )

## Setting up Constraints when using the Declarative ORM Extension

The `Table` is the SQLAlchemy Core construct that allows one to define table metadata, which among other things can be used by the SQLAlchemy ORM as a target to map a class. The Declarative extension allows the `Table` object to be created automatically, given the contents of the table primarily as a mapping of `Column` objects.

To apply table-level constraint objects such as `ForeignKeyConstraint` to a table defined using Declarative, use the `__table_args__` attribute, described at Table Configuration.

## Constraints API

class `sqlalchemy.schema.` `Constraint`(name=None, deferrable=None, initially=None, _create_rule=None, **kw)

  Bases: `sqlalchemy.schema.SchemaItem`

  A table-level SQL constraint.

class `sqlalchemy.schema.` `CheckConstraint`(sqltext, name=None, deferrable=None, initially=None, table=None, _create_rule=None, _autoattach=True)

  Bases: `sqlalchemy.schema.Constraint`

  A table- or column-level CHECK constraint.

  Can be included in the definition of a Table or Column.

class `sqlalchemy.schema.` `ColumnCollectionConstraint`(*columns, **kw)

  Bases: `sqlalchemy.schema.ColumnCollectionMixin`, `sqlalchemy.schema.Constraint`

  A constraint that proxies a ColumnCollection.

class `sqlalchemy.schema.` `ForeignKey`(column, _constraint=None, use_alter=False, name=None, onupdate=None, ondelete=None, deferrable=None, schema=None, initially=None, link_to_name=False, match=None)

  Bases: `sqlalchemy.schema.SchemaItem`

  Defines a dependency between two columns.

  `ForeignKey` is specified as an argument to a `Column` object, e.g.:

      t = Table("remote_table", metadata,
          Column("remote_id", ForeignKey("main_table.id"))
      )

  Note that `ForeignKey` is only a marker object that defines a dependency between two columns. The actual constraint is in all cases represented by the `ForeignKeyConstraint` object. This object will be generated automatically when a `ForeignKey` is associated with a `Column` which in turn is associated with a `Table`. Conversely, when `ForeignKeyConstraint` is applied to a `Table`, `ForeignKey` markers are automatically generated to be present on each associated `Column`, which are also associated with the constraint object.

  Note that you cannot define a “composite” foreign key constraint, that is a constraint between a grouping of multiple parent/child columns, using `ForeignKey` objects. To define this grouping, the `ForeignKeyConstraint` object must be used, and applied to the `Table`. The associated `ForeignKey` objects are created automatically.

  The `ForeignKey` objects associated with an individual `Column` object are available in the foreign_keys collection of that column.

  Further examples of foreign key configuration are in metadata_foreignkeys.

  `__init__`(column, _constraint=None, use_alter=False, name=None, onupdate=None, ondelete=None, deferrable=None, schema=None, initially=None, link_to_name=False, match=None)

    Construct a column-level FOREIGN KEY.

    The `ForeignKey` object when constructed generates a `ForeignKeyConstraint` which is associated with the parent `Table` object’s collection of constraints.

    Parameters: | column – A single target column for the key relationship. A `Column` object or a column name as a string: `tablename.columnkey` or `schema.tablename.columnkey`. `columnkey` is the `key` which has been assigned to the column (defaults to the column name itself), unless `link_to_name` is `True` in which case the rendered name of the column is used. New in version 0.7.4: Note that if the schema name is not included, and the underlying `MetaData` has a “schema”, that value will be used. name – Optional string. An in-database name for the key if constraint is not provided. onupdate – Optional string. If set, emit ON UPDATE <value> when issuing DDL for this constraint. Typical values include CASCADE, DELETE and RESTRICT. ondelete – Optional string. If set, emit ON DELETE <value> when issuing DDL for this constraint. Typical values include CASCADE, DELETE and RESTRICT. deferrable – Optional bool. If set, emit DEFERRABLE or NOT DEFERRABLE when issuing DDL for this constraint. initially – Optional string. If set, emit INITIALLY <value> when issuing DDL for this constraint. link_to_name – if True, the string name given in `column` is the rendered name of the referenced column, not its locally assigned `key`. use_alter – passed to the underlying `ForeignKeyConstraint` to indicate the constraint should be generated/dropped externally from the CREATE TABLE/ DROP TABLE statement. See that classes’ constructor for details. match – Optional string. If set, emit MATCH <value> when issuing DDL for this constraint. Typical values include SIMPLE, PARTIAL and FULL. schema – Deprecated; this flag does nothing and will be removed in 0.9.

  `column`

    Return the target `Column` referenced by this `ForeignKey`.

    If this `ForeignKey` was created using a string-based target column specification, this attribute will on first access initiate a resolution process to locate the referenced remote `Column`. The resolution process traverses to the parent `Column`, `Table`, and `MetaData` to proceed - if any of these aren’t yet present, an error is raised.

  `copy`(schema=None)

    Produce a copy of this `ForeignKey` object.

    The new `ForeignKey` will not be bound to any `Column`.

    This method is usually used by the internal copy procedures of `Column`, `Table`, and `MetaData`.

    Parameters: | schema – The returned `ForeignKey` will reference the original table and column name, qualified by the given string schema name.

  `get_referent`(table)

    Return the `Column` in the given `Table` referenced by this `ForeignKey`.

    Returns None if this `ForeignKey` does not reference the given `Table`.

  `references`(table)

    Return True if the given `Table` is referenced by this `ForeignKey`.

  `target_fullname`

    Return a string based ‘column specification’ for this `ForeignKey`.

    This is usually the equivalent of the string-based “tablename.colname” argument first passed to the object’s constructor.

class `sqlalchemy.schema.` `ForeignKeyConstraint`(columns, refcolumns, name=None, onupdate=None, ondelete=None, deferrable=None, initially=None, use_alter=False, link_to_name=False, match=None, table=None)

  Bases: `sqlalchemy.schema.Constraint`

  A table-level FOREIGN KEY constraint.

  Defines a single column or composite FOREIGN KEY ... REFERENCES constraint. For a no-frills, single column foreign key, adding a `ForeignKey` to the definition of a `Column` is a shorthand equivalent for an unnamed, single column `ForeignKeyConstraint`.

  Examples of foreign key configuration are in metadata_foreignkeys.

  `__init__`(columns, refcolumns, name=None, onupdate=None, ondelete=None, deferrable=None, initially=None, use_alter=False, link_to_name=False, match=None, table=None)

    Construct a composite-capable FOREIGN KEY.

    Parameters: | columns – A sequence of local column names. The named columns must be defined and present in the parent Table. The names should match the `key` given to each column (defaults to the name) unless `link_to_name` is True. refcolumns – A sequence of foreign column names or Column objects. The columns must all be located within the same Table. name – Optional, the in-database name of the key. onupdate – Optional string. If set, emit ON UPDATE <value> when issuing DDL for this constraint. Typical values include CASCADE, DELETE and RESTRICT. ondelete – Optional string. If set, emit ON DELETE <value> when issuing DDL for this constraint. Typical values include CASCADE, DELETE and RESTRICT. deferrable – Optional bool. If set, emit DEFERRABLE or NOT DEFERRABLE when issuing DDL for this constraint. initially – Optional string. If set, emit INITIALLY <value> when issuing DDL for this constraint. link_to_name – if True, the string name given in `column` is the rendered name of the referenced column, not its locally assigned `key`. use_alter – If True, do not emit the DDL for this constraint as part of the CREATE TABLE definition. Instead, generate it via an ALTER TABLE statement issued after the full collection of tables have been created, and drop it via an ALTER TABLE statement before the full collection of tables are dropped. This is shorthand for the usage of `AddConstraint` and `DropConstraint` applied as “after-create” and “before-drop” events on the MetaData object. This is normally used to generate/drop constraints on objects that are mutually dependent on each other. match – Optional string. If set, emit MATCH <value> when issuing DDL for this constraint. Typical values include SIMPLE, PARTIAL and FULL.

class `sqlalchemy.schema.` `PrimaryKeyConstraint`(*columns, **kw)

  Bases: `sqlalchemy.schema.ColumnCollectionConstraint`

  A table-level PRIMARY KEY constraint.

  Defines a single column or composite PRIMARY KEY constraint. For a no-frills primary key, adding `primary_key=True` to one or more `Column` definitions is a shorthand equivalent for an unnamed single- or multiple-column PrimaryKeyConstraint.

class `sqlalchemy.schema.` `UniqueConstraint`(*columns, **kw)

  Bases: `sqlalchemy.schema.ColumnCollectionConstraint`

  A table-level UNIQUE constraint.

  Defines a single column or composite UNIQUE constraint. For a no-frills, single column constraint, adding `unique=True` to the `Column` definition is a shorthand equivalent for an unnamed, single column UniqueConstraint.

## Indexes

Indexes can be created anonymously (using an auto-generated name `ix_<column label>`) for a single column using the inline `index` keyword on `Column`, which also modifies the usage of `unique` to apply the uniqueness to the index itself, instead of adding a separate UNIQUE constraint. For indexes with specific names or which encompass more than one column, use the `Index` construct, which requires a name.

Below we illustrate a `Table` with several `Index` objects associated. The DDL for “CREATE INDEX” is issued right after the create statements for the table:

    meta = MetaData()
    mytable = Table('mytable', meta,
        # an indexed column, with index "ix_mytable_col1"
        Column('col1', Integer, index=True),

        # a uniquely indexed column with index "ix_mytable_col2"
        Column('col2', Integer, index=True, unique=True),

        Column('col3', Integer),
        Column('col4', Integer),

        Column('col5', Integer),
        Column('col6', Integer),
        )

    # place an index on col3, col4
    Index('idx_col34', mytable.c.col3, mytable.c.col4)

    # place a unique index on col5, col6
    Index('myindex', mytable.c.col5, mytable.c.col6, unique=True)

    sqlmytable.create(engine)
    CREATE TABLE mytable (
        col1 INTEGER,
        col2 INTEGER,
        col3 INTEGER,
        col4 INTEGER,
        col5 INTEGER,
        col6 INTEGER
    )
    CREATE INDEX ix_mytable_col1 ON mytable (col1)
    CREATE UNIQUE INDEX ix_mytable_col2 ON mytable (col2)
    CREATE UNIQUE INDEX myindex ON mytable (col5, col6)
    CREATE INDEX idx_col34 ON mytable (col3, col4)

Note in the example above, the `Index` construct is created externally to the table which it corresponds, using `Column` objects directly. `Index` also supports “inline” definition inside the `Table`, using string names to identify columns:

    meta = MetaData()
    mytable = Table('mytable', meta,
        Column('col1', Integer),

        Column('col2', Integer),

        Column('col3', Integer),
        Column('col4', Integer),

        # place an index on col1, col2
        Index('idx_col12', 'col1', 'col2'),

        # place a unique index on col3, col4
        Index('idx_col34', 'col3', 'col4', unique=True)
    )

New in version 0.7: Support of “inline” definition inside the `Table` for `Index`.

The `Index` object also supports its own `create()` method:

    i = Index('someindex', mytable.c.col5)
    sqli.create(engine)
    CREATE INDEX someindex ON mytable (col5)

### Functional Indexes

`Index` supports SQL and function expressions, as supported by the target backend. To create an index against a column using a descending value, the `ColumnElement.desc()` modifier may be used:

    from sqlalchemy import Index

    Index('someindex', mytable.c.somecol.desc())

Or with a backend that supports functional indexes such as Postgresql, a “case insensitive” index can be created using the `lower()` function:

    from sqlalchemy import func, Index

    Index('someindex', func.lower(mytable.c.somecol))

New in version 0.8: `Index` supports SQL expressions and functions as well as plain columns.

## Index API

class `sqlalchemy.schema.` `Index`(name, *expressions, **kw)

  Bases: `sqlalchemy.schema.ColumnCollectionMixin`, `sqlalchemy.schema.SchemaItem`

  A table-level INDEX.

  Defines a composite (one or more column) INDEX.

  E.g.:

      sometable = Table("sometable", metadata,
                      Column("name", String(50)),
                      Column("address", String(100))
                  )

      Index("some_index", sometable.c.name)

  For a no-frills, single column index, adding `Column` also supports `index=True`:

      sometable = Table("sometable", metadata,
                      Column("name", String(50), index=True)
                  )

  For a composite index, multiple columns can be specified:

      Index("some_index", sometable.c.name, sometable.c.address)

  Functional indexes are supported as well, keeping in mind that at least one `Column` must be present:

      Index("some_index", func.lower(sometable.c.name))

  New in version 0.8: support for functional and expression-based indexes.

  See also

  Indexes - General information on `Index`.

  Postgresql-Specific Index Options - PostgreSQL-specific options available for the `Index` construct.

  MySQL Specific Index Options - MySQL-specific options available for the `Index` construct.

  MSSQL-Specific Index Options - MSSQL-specific options available for the `Index` construct.

  `__init__`(name, *expressions, **kw)

    Construct an index object.

    Parameters: | name – The name of the index *expressions – Column or SQL expressions. unique – Defaults to False: create a unique index. **kw – Other keyword arguments may be interpreted by specific dialects.

  `bind`

    Return the connectable associated with this Index.

  `create`(bind=None)

    Issue a `CREATE` statement for this `Index`, using the given `Connectable` for connectivity.

    See also

    `MetaData.create_all()`.

  `drop`(bind=None)

    Issue a `DROP` statement for this `Index`, using the given `Connectable` for connectivity.

    See also

    `MetaData.drop_all()`.